Googling didn't reveal many people using the MOE SL handguard with railed gas blocks. There was one other poster here who mentioned interference with his railed gas block and the MOE SL handgauard and he was able to fix it by drum sanding away a portion of the handguard. Looks like I'm having the same experience. I couldn't install the SL handguared with my upper height railed gas block. Because of the way the gas block hits the inside of the upper half of the hand guard you can't push the front locking tabs far enough into the end cap to swing the rear down and clear the delta ring. I needed another 0.094" of clerance to be able to swing the back end under the delta ring. I got creative, loosened my gas block and slide it forward in order to get the top mounted. Then I was able to push my gas block into place and tighten it back down in order to install it. Even doing it this way the upper portion of the hand guard did not allow me to push the gas block firmly against the end cap. I had to remove about 0.030 of material in order to get it to seat all the way. Love the handguard,by far my favorite drop in hand guard, but if you are using a railed gas block be prepared to do a little work to get it installed.